feat: support CRAM-MD5 authentication (RFC 2195)#11
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Advertise and accept AUTH CRAM-MD5 alongside PLAIN and LOGIN. The existing [[server.auth]] username/password credentials serve all three mechanisms, so no config changes are needed. - smtp: new AuthenticatingCramMd5 session state, challenge generation (<counter.nanos@hostname>, unique per session to prevent digest replay), and an on_auth_cram_md5 callback with a supports_cram_md5 capability gate so implementations that don't support the mechanism neither advertise it nor accept it (504 instead). - hedwig: verify the client digest by recomputing HMAC-MD5(password, challenge) from the stored password, compared in constant time. hmac and md-5 were already in the tree as transitive deps. - Harden the AUTH exchange for all mechanisms: cancellation with '*' (RFC 4954) and malformed base64/response data now get a 501 and return the session to Greeted instead of tearing down the connection. Verified end-to-end with Python smtplib forcing CRAM-MD5 against a live server: 235 on correct password, 535 on wrong, mail accepted after auth.
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Summary
Adds
AUTH CRAM-MD5support alongside the existing PLAIN and LOGIN mechanisms. No config changes needed — the existing[[server.auth]]username/password credentials serve all three mechanisms, since CRAM-MD5 verification just recomputes the HMAC from the stored plaintext password. The mechanism is negotiated per-connection by the client, so there is no per-credentialauth_typeknob.smtp crate
AuthCramMd5/AuthCramMd5Responsecommands andAuthenticatingCramMd5(challenge)session state.<counter.nanos@hostname>, using the configurable hostname from feat: make inbound SMTP greeting/EHLO hostname configurable #9). The atomic counter + timestamp guarantees per-session uniqueness — which is what blocks digest replay — without adding a rand dependency.on_auth_cram_md5(username, challenge, digest)trait callback plus asupports_cram_md5()capability gate (both defaulted, non-breaking): implementations that don't opt in neither advertise CRAM-MD5 in EHLO nor accept it (504instead).hedwig
on_auth_cram_md5recomputesHMAC-MD5(password, challenge)and compares digests in constant time. Lowercase hex per the RFC; uppercase tolerated.hmacandmd-5added as direct deps — both were already in the tree transitively.AUTH exchange hardening (all mechanisms)
*(RFC 4954 §4) now gets501and returns the session toGreetedinstead of tearing down the connection.501instead of a connection-killing internal error. Both flaws pre-existed in the PLAIN/LOGIN paths and are fixed uniformly.Note
CRAM-MD5 is a legacy-client compatibility feature, not a security upgrade: it avoids sending the password in cleartext, but MD5-based challenge-response is obsolete and PLAIN over TLS remains the stronger option.
Testing
535, cancellation, malformed responses,504when the callback doesn't support the mechanism, recovery after malformedAUTH PLAIN.smtplibforcing CRAM-MD5:235on correct password,535on wrong, mail accepted after auth.